Yesterday was the best day to sell your iPhone in hopes of upgrading to the next-gen model, what we now know as the iPhone 4S. But if you were cautious (and rightly so) about selling your current model when the next generation remained up in the air, you still have options. And surprisingly, you might be able to cash in for nearly as much — or in some cases more.

Cash-for-gadgets outfit Gazelle tells us that today has been the biggest iPhone trade-in day the site has ever seen, despite any general disappointment over the iPhone 4S. Today alone, Gazelle has had over 2,500 trades and expects to receive 30,000 total in 10 days — which is how many it amassed over about two months when people traded in their last-gen units for the iPhone 4 last year.

Sprint members out there are getting in on the action too, and there's been a big increase in Android and BlackBerry Sprint models hitting the site.

This is all good news for Apple upgrade numbers, and surprisingly for consumers as well. Be aware, however, that there is no time like the present: Resale value has fluctuated along with the announcement but is expected to sink from here on out.

eBay Instant Sale

eBay tells us it will be offering a temporary Instant Sale promotion that ends tomorrow at midnight. You will be able to get up to $357.50 for a variety of phones (not only iPhones) and get cash in return. Here are a few other phones that can help you save (or completely pay for) for the iPhone 4S.

Verizon iPhone 4: Up to $318

AT&T iPhone 3GS: Up to $229

HTC EVO 3D: Up to $200

HTC Thunderbolt: Up to $200

HTC Droid Incredible 2: Up to $200

HTC Sensation 4G: Up to $225

Even the iPhone 3GS is still managing to pull in as much as $229 on the site. Not bad, considering yesterday prices were slightly lower.

You'll be happy to hear there is more good news. Gazelle tells us you can sell your 32GB AT&T iPhone for $249, only a dollar less than yesterday. And a 32GB Verizon iPhone is actually worth more today: $238.

The iPhone 4 has gone down a little in value over at NextWorth. You can now sell your 32GB AT&T model for $215—$172 if you're with Verizon. A 32GB 3GS can be cashed in for $165.

Craigslist

After doing a bit of searching around Craigslist, we saw people willing to pay as much as $350 for an iPhone 4 and $200 for an iPhone 3GS. We hate to say it, but Craigslist may be your best bet, simply because consumers are naming their own prices instead of resale sites, and it seems like at least some of them aren't doing their homework (search for "iPhone 4G"). That said, the bigger the city, the better your odds of finding an ill-informed buyer. And you're the one that has to live with the guilt.

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